Probably you thought about this already, but have you repeated the Ca increase in different types of container (metal, glass, plastic, wood)?
The basic idea is to use a container that can’t add anything that one intends to measure, obviously, and the choice in this case is PP beakers, which is the same used by Lu et al. PP can only add C and O, and that is sampled with the distilled water “blank”. Wood would introduce a lot of potential contaminants so is discarded from start. Stainless steel could also be used but it’s always preferred something transparent for observing the reaction. Glass is not recommended as it can also add contaminants and can always be a security hazard when used with a sonotrode.